The present invention relates generally to the field of ceramics and particularly to the use of magnesium oxide as a sintering aid in the preparation of fully dense ceramic articles by the powder metallurgy processing of silicon nitride and aluminum oxide compositions.
The high density ceramic articles of the present invention have utility as structural materials having good thermal shock resistance which are suitable for use where exceptional strength is not a critical requirement of the material.
Hot pressed silicon nitride (Si.sub.3 N.sub.4) has been the subject of considerable investigation by a number of researchers because of its utility as a high temperature structural material. These investigations have encountered certain problems in the fabrication of Si.sub.3 N.sub.4 articles, one of which is the difficulty of hot pressing the material to high density without the use of a sintering aid. For example, see British Pat. No. 1,092,637 and U.S. Pat. No. 3,821,005.
Recently, Jack and Wilson, in Nature Physical Science, Vol. 238, pp. 28 and 29, July 10, 1972, have reported that ceramics based on Si.sub.3 N.sub.4 - Al.sub.2 O.sub.3 (Si-Al-O-N) solid solution compositions hold great promise because the compositions result in a material with a thermal expansion lower than that of hot-pressed Si.sub.3 N.sub.4. Jack and Wilson have reported further that the Si-Al-O-N system can be sintered to maximum density by conventional ceramic techniques, e.g. slip-casting, extrusion or pressing to produce pre-fired shapes which are heated to their sintering temperature without the application of pressure during the heating stage. These conventional sintering techniques are to be distinguished from the process known as "hot-pressing" which involves the application of extreme pressure during the sintering operation.
A process involving the hot-press sintering of Si.sub.3 N.sub.4 -Al.sub.2 O.sub.3 compositions is described in German Offen.2,262,785. Other refractory ceramic articles composed of Si.sub.3 N.sub.4 and metal oxides, particularly aluminum oxide, have been prepared according to the processes disclosed in Japanese Pat. No. 9,476 (1969) and Japanese Kokai 79,216 (1973). However, none of the aforementioned prior art processes discloses the novel features of the instant invention.
In an attempt to fabricate fully dense ceramic articles having low thermal expansion by conventional processing techniques, as reported by Jack and Wilson, it was discovered that the density and the thermal expansion of the articles produced from the Si-Al-O-N compositions were inferior to the density and thermal expansion of hot-pressed Si.sub.3 N.sub.4.
Thus the problem of fabricating a fully dense ceramic article which has good thermal shock resistance from a Si.sub.3 N.sub.4 - Al.sub.2 O.sub.3 composite powder composition by heating the composition to its sintering temperature without the need for exerting pressure on the composition during the sintering operation remains unsolved when prior art processing methods are employed.